Using Exchange, my whole INBOX and SENT ITEMS mailfolder tree dispalys in Mail,most of them emty. How do I delete or colapse them? Anybody that can help?

To just not see them:

In landscape mode, tap on the Mailboxes button at the top left corner. Select your Inbox, or All Inboxes, whichever applies.

If you want to see the folders again, do the same thing but select the account instead.

In order to permanently remove folder, you will have to go to your Exchange account on the computer or through a web browser and remove the folders there. Make sure there isn't anything in them you want to keep first.

If there is a way to collapse subfolders, I haven't found it.